Control of stoichiometry, microstructure, and mechanical properties in SiC coatings produced by fluidized bed chemical v
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Stoichiometric silicon carbide coatings the same as those used in the formation of TRISO (TRistructural ISOtropic) fuel particles were produced by the decomposition of methyltrichlorosilane in hydrogen. Fluidized bed chemical vapor deposition at around 1500 °C, produced SiC with a Young’s modulus of 362 to 399 GPa. In this paper we demonstrate the deposition of stoichiometric silicon carbide coatings with refined microstructure (grain size between 0.4 and 0.8 m) and enhanced mechanical properties (Young’s modulus of 448 GPa and hardness of 42 GPa) at 1300 °C by the addition of propene. The addition of ethyne, however, had little effect on the deposition of silicon carbide. The effect of deposition temperature and precursor concentration were correlated to changes in the type of molecules participating in the deposition mechanism. I. INTRODUCTION
Among the different applications of silicon carbide (SiC), its use as a coating material in the so-called TRISO (TRistructural ISOtropic) coated fuel particle has become increasingly important due to the growing interest of different countries in developing high-temperature nuclear reactors (HTR). The structural integrity and performance of the coated fuel is the most important safety aspect in this nuclear reactor, since it relies on the properties of the coatings surrounding the kernel fuel to stop the release of harmful radioactive material.1,2 This coated fuel particle, made up of three layers of pyrolytic carbon and one of SiC, is produced by fluidized bed chemical vapor deposition (FBCVD).3 Among these layers, the silicon carbide coating is considered the most important since it provides the mechanical strength and is responsible for retaining most of the solid fission products.4,5 Because of the importance of silicon carbide, extensive work has been carried out studying the effect of deposition temperature and precursor concentration on the microstructure and stoichiometry of this material, especially for the case of methyltrichlorosilane (MTS) as precursor.4,6–11 Generally, it was observed that stoichiometric SiC was produced between 1500 and 1600 °C, with silicon or carbon codeposited below or above these temperatures, respectively. In addition to the codeposition of Si, porosity was also found to be formed for some cases at deposition temperatures below 1500 °C. Grain
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0220 J. Mater. Res., Vol. 23, No. 6, Jun 2008
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size, on the other hand, was observed to decrease as deposition temperature or precursor concentration was reduced. For these reasons typical SiC coatings are generally produced between 1500 and 1600 °C with grain sizes between 1 and 8 m and with coating rates of around 0.2 m/min.5,12 Furthermore, the Young’s modulus of this material obtained by nanoindentation has been found to be between 340 and 370 GPa.13,14 The use of a precracked microcantilever gave a value of Young’s modulus of 396 GPa.
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